As is well known, a pipe expanding mandrel of this type requires change in the size of the pipe expanding billet at the front end of the mandrel according to a desired change in the diameter of the pipe to be expanded. However, replacing the entire mandrel each time when there is a change in desired pipe diameter involves removal of the mandrel from and mounting another mandrel to the pipe expanding apparatus. Such mounting and removing operations are very troublesome and time-consuming.
In order to facilitate billet replacing operations according to desired changes in the required billet size, there has hitherto been often used means such as that illustrated in FIG. 6, for example, wherein a pin 2a inserted through a pipe expanding billet 3 is brought in mating engagement with an internally threaded portion 30 formed in a front end portion of a mandrel shaft 13a, the billet 3 being firmly fixed in position by the pin 2a, whereby the billet 3 alone can be replaced through removal of the pin 2a without requiring the replacement of the entire pipe expanding mandrel.
With such conventional means, however, it is necessary that in replacing the pipe expanding billet 3, the pin 2a which is in threaded engagement with the mandrel shaft 13a be disengaged from the mandrel shaft 13a by rotation of the pin 2a. Thus, the prior art means still involves a drawback from the standpoint of ease of operation because the disengagement and engagement operations required with respect to the pin 2a are troublesome and time-consuming, though the means provides some improvement over the earlier practice which required replacement of the entire mandrel.